Officially you are supposed to register the car in Cali within 30 days.

If you bought it in a state with lower tax rate than Cali, Cali DMV will collect the difference, unless you owned it for at least a year.

So if you buy a van in oregon, no tax then "import" it into Cali, California will want 9=10% tax...unless you can prove you owned it for over a year as an out of state resident.

BTW, cali sales taxes and DMV fees go up April 1st, no fooling....